1. Field of the Invention
The present invention relates to a zoom lens system suitable for use in a single lens reflex (SLR) digital camera, and in particular, relates to a standard zoom lens system having the range of an angle-of-view including an angle-of-view of approximately 45° (a standard lens system) which is equivalent to 50 mm in the case of an SLR camera with a 35 mm-film format.
2. Description of the Prior Art
As standard zoom lens systems, a negative-lead type zoom lens system and a positive-lead type zoom lens system are known in the art.
The negative-lead type zoom lens system is constituted by, e.g., a lens group having a negative refractive power (hereinafter, a negative lens group) and a lens group having a positive refractive power (hereinafter, a positive lens group), in this order from the object, or, a negative lens group, positive lens group, a negative lens group and a positive lens group construction, in this order from the object.
The positive-lead type zoom lens system is constituted by, e.g., a positive lens group, a negative lens group, a positive lens group and a positive lens group construction, in this order from the object.
The negative-lead type zoom lens system has at least the following advantages: (i) the production cost is low, (ii) the mechanical structure is simple, and (iii) the negative-lead type zoom lens system is easy to be manufactured due to low sensitivity.
On the other hand, the negative-lead type zoom lens system has the following disadvantages: (i) the zoom ratio cannot be made large, (ii) the fluctuation of F-number upon zooming is large, and (iii) the front lens group is large and heavy since focusing is performed by the front lens group.
The positive-lead type zoom lens system has at least the following advantages: (i) the zoom ratio can be made large, (ii) the fluctuation of F-number upon zooming is small, and (iii) the focusing lens group can be made light (less heavy) by employing an internal focusing lens system.
On the other hand, the positive-lead type zoom lens system has the following disadvantages: (i) the production cost is high, (ii) the mechanical structure is complex, and (iii) the positive-lead type zoom lens system is difficult to be manufactured due to high sensitivity.
Examples of the prior art can be found in Japanese Unexamined Patent Publication Nos. 2002-6217, 2003-50350, 2004-37921 and 2004-333770.